The use of copper as a conductive interconnect material is favored in semiconductor devices because of the high speed that copper provides. Copper interconnect structures are typically formed using damascene processing technology in which an opening is formed in a dielectric layer, copper is deposited within the opening, and then a polishing/planarization process is used to remove copper from over the dielectric layer, leaving copper inlaid within the opening. However, copper diffuses through dielectric materials, so the copper interconnect structures must be encapsulated by a diffusion barrier layer. Otherwise the diffused copper metal in the dielectric layer may result in current leakage between the interconnect structures. The diffusion barrier layer typically comprises a refractory material. Typical refractory materials used for the barrier layer include tantalum (Ta), tantalum nitride (TaN), titanium (Ti), and titanium nitride (TiN).
Studies have concluded that as the critical dimensions of integrated circuits (IC's) continue to shrink, seams or cracks may develop at a joinder between the copper and the diffusion barrier layer. These seams or cracks lead to the phenomena known as “electromigration” (EM) and “stress migration” that degrades the reliability of the copper interconnect structure.